Process and apparatus for the production of carbon nanotubes

ABSTRACT

A process for preparing carbon nanotubes comprising locating a substrate ( 1 ) capable of supporting carbon nanotube growth in a localised heating zone within a reaction chamber ( 7 ), said localised heating zone being provided by a heating element ( 2 ) located within the reaction chamber ( 7 ), passing a gaseous carbonaceous material into the reaction chamber ( 7 ) such that the gaseous material passes over and contacts the substrate ( 1 ) in the localised heating zone, whereby the gaseous material undergoes pyrolysis under the influence of the heat to form carbon nanotubes on the substrate ( 1 ). Embodiments of the process prepare multilayer carbon nanotubes and hetero-structured multilayer carbon nanotube films.

[0001] This invention relates to carbon nanotubes, in particular to aprocess and apparatus for the preparation of carbon nanotubes.

[0002] Carbon nanotubes usually have a diameter in the order of 0.4nanometres to 100 nanometres and a length of up to about 1 centimetre.These elongated nanotubes consist of carbon hexagons arranged in aconcentric manner with both ends of the tubes normally capped bypentagon-containing fullerene-like structures. Carbon nanotubes may havea single wall or multiwall structure. They can behave as a semiconductoror metal depending on their diameter and helicity of the arrangement ofgraphitic rings in the walls, and dissimilar carbon nanotubes may bejoined together allowing the formation of molecular wires withinteresting electrical, magnetic, nonlinear optical, thermal andmechanical properties. These unusual properties have led to diversepotential applications for carbon nanotubes in material science andnanotechnology. Indeed, carbon nanotubes have been proposed as newmaterials for electron field emitters in panel displays,single-molecular transistors, scanning probe microscope tips, gas andelectrochemical energy storages, catalyst and proteins/DNA supports,molecular-filtration membranes, and energy-absorbing materials (see, forexample: M. Dresselhaus, et al., Phys. World, Jan. 33, 1998; P.M.Ajayan, and T. W. Ebbesen, Rep. Prog. Phys., 60,1027, 1997; R. Dagani,C&E News, Jan. 11, 31, 1999). The importance of carbon nanotechnology isevidenced by increasing research and development funding (C & E News,May 1, 2000, pp.41-47).

[0003] For most of the above applications, it is highly desirable thatthe carbon nanotubes are aligned and/or formed into patterns so that theproperties of the individual nanotubes can be easily assessed and theycan be incorporated effectively into devices.

[0004] Carbon nanotubes have been synthesised using arc discharge (S.Iijima, Nature, 354, 56-68, 1991; T. W. Ebbesen and P. M. Aegean,Nature, 358, 220-222, 1992) and catalytic pyrolysis (see, for example:M. End. et Aa J. Pays Chum. Solids, 54, 1841-1848, 1994; V. Ivanov, etAl Chum. Pays. Let. 223, 329-335, 1994) and often exist in an randomlyentangled state. Patterned and non-patterned carbon nanotube filmshaving the nanotubes aligned perpendicularly with the substrate havebeen prepared by pyrolysis of iron (II) phthalocyanine in a flow reactorcomprising a quartz glass tube heated by a dual furnace (J. Phys. Chem.B., 104, 2000, 1891). Ren et al., have synthesised large arrays ofwell-aligned carbon nanotubes by radio-frequency sputter-coating of athin nickel layer onto a substrate, followed by plasma-enhanced hotfilament chemical vapour deposition of acetylene in the presence ofammonia gas at approximately 666° C. (Science, 282, 1998, 1105).

[0005] Carbon nanotubes may be prepared at a variety of temperatures,although generally higher temperatures, for example, 600° C. to 1100°C., are required for the preparation of aligned carbon nanotubes. Foreconomic reasons it is preferable to prepare carbon nanotubes at lowertemperatures, for example, between 300° C. to 800° C.

[0006] Carbon nanotubes can be prepared in flow reactors comprising aglass tube surrounded by a dual furnace. This technique results in theentire reactor, including the glass tube, being heated and maintained atpyrolysis temperature. Furthermore, carbon is not only deposited on thesubstrate, but also on the other hot surfaces in the reactor, such asthe inside of the glass tube. The carbon deposits on the glass canobscure the view of the substrate, making it difficult to visuallymonitor the growth of the nanotubes. The positioning of the furnace alsogenerally obscures the view of the substrate and the growth of thenanotubes.

[0007] It is an object of the present invention to overcome or at leastalleviate one or more of the disadvantages of the prior art.

[0008] According to a first aspect of the invention there is provided aprocess for preparing carbon nanotubes comprising:

[0009] locating a substrate capable of supporting carbon nanotube growthin a localised heating zone within a reaction chamber, said localisedheating zone being provided by a heating element located within saidreaction chamber,

[0010] passing a gaseous carbonaceous material into said reactionchamber such that the gaseous material passes over and contacts saidsubstrate in the localised heating zone, whereby said gaseous materialundergoes pyrolysis under the influence of said heat to form carbonnanotubes on said substrate.

[0011] According to a second aspect of the invention there is provided areactor for preparing carbon nanotubes comprising:

[0012] a reaction chamber,

[0013] at least one support means located within said reaction chambercapable of supporting a substrate, said substrate being capable ofsupporting carbon nanotube growth,

[0014] at least one heating element located within said reaction chambercapable of providing localised heating to said substrate within saidreaction chamber,

[0015] means for passing a gaseous carbonaceous material into saidreaction chamber such that it passes over and contacts said substrate.

[0016] According to the present invention the substrate is heated by aheating element in a localised heating zone within a reaction chamber,thereby avoiding the need to heat the entire reaction chamber topyrolysis temperatures. While the pyrolysis can be achieved at anysuitable temperature in the localised heating zone, the process of theinvention conveniently allows the preparation of carbon nanotubes attemperatures as low as 300° C. The carbon nanotubes are grown on asubstrate that is heated to the required temperature by the heatingelement. In view of the lower temperatures required and the fact thatthe heating is localised, the present invention can provide substantialenergy and cost savings relative to conventional methods. Also, sincethe heating is localised to the heating zone, the growth of carbonnanotubes at sites within the reaction chamber other than on thesubstrate and the production of amorphous carbon byproducts inside thereaction chamber are minimised. This also leads to a cleaner reactionchamber and purer carbon nanotube films being formed. If amorphouscarbon is deposited on other hot surfaces, for example, exposed areas ofthe heating element, they are readily removed by heating the heatingelement in air, causing the amorphous carbon to be oxidised to CO₂. Thereaction chamber therefore may be easily cleaned.

[0017] The reaction chamber may be defined by one or more walls, and maybe of any size or shape suitable for accommodating one or more heatingelements. The wall(s) may be formed from any suitable material,including metals, such as steel, aluminium, copper, silver, platinum oralloys, glass, such as quartz glass, normal glass or the like, plastic,polymethylmethacrylate (PMMA), Mylar, polypropylene (PP), polyethylene(PE) or their composites, or a combination thereof. The localisedheating zone in the vicinity of the heating element ensures that thetemperature of the wall(s) of the chamber remain lower than thetemperature in the localised heating zone where pyrolysis occurs. If thechamber is large enough, walls of the chamber remote from the heatingzone will remain at ambient temperature. Preferably the reaction chamberis formed from glass or at least includes one or more glass panels.Preferably, the chamber is tubular and its walls are transparent orpartially transparent. Advantageously, transparency allows the visualobservation of carbon nanotube growth and therefore undesirable growthcan be terminated at any stage. Visual observation also allows easiercontrol of the length of the carbon nanotubes by stopping the growthprocess at a desired time.

[0018] The support means may be any support means capable of supportinga substrate within the reaction chamber and capable of withstanding thepyrolysis temperatures used. For example, the support means may be inthe form of a solid block, plate, grate, bracket, cradle, stretcher,scaffold or the like and may be made from any suitable material, forexample, metal or ceramic materials. The support means may be any sizeor shape suitable to support the substrate.

[0019] The heating element may be any suitable heating means capable ofheating a substrate and providing a localised heating zone. For example,suitable heating means may include resistant wires, induction field,microwave radiation or infrared radiation. The localised heating zonecan also be heated from a remote point by, for example, a focussedinfrared beam or laser beam. In a preferred embodiment, the heatingelement also acts as the support means for the substrate. In thisembodiment, the heating element preferably forms a flat surface uponwhich the substrate is supported. An example of a suitable heatingelement which also acts as a substrate support is a ceramic plate intowhich resistant wires have been inserted. The heating element/substratesupport may be formed in any shape or size appropriate to support andheat the substrate and heating zone. Preferably, the heating elementallows the substrate to be heated homogenously, i.e., the temperaturedistribution of the heated substrate is homogenous. One means ofachieving homogenous temperature distribution is to place a conductingmaterial, for example, a copper sheet, between the heating plate and thesubstrate allowing even temperature distribution.

[0020] The reactor of the invention also includes a means for passinggaseous carbonaceous material into the reaction chamber such that itpasses over and contacts the substrate. This means may be provided by atleast one gas conduit. The at least one gas conduit is positioned toallow the flow of gaseous carbonaceous material into the localisedheating zone. In a preferred embodiment, the inlet for the carbonaceousmaterial is positioned directly above the substrate so that the gaseouscarbonaceous material is supplied directly to the localised heatingzone. Alternatively, the gaseous carbonaceous material may be suppliedthrough an inlet at one end of the chamber and allowed to flow acrossthe substrate in the localised heating zone. Multiple gas conduits maybe used to supply gaseous carbonaceous material to a large localisedheating zone or multiple localised heating zones located within thereaction chamber.

[0021] The at least one gas conduit may also be used as a gas inlet forsupplying other gases to the reaction chamber and as a gas outlet toallow the exit of gases from the chamber. One gas conduit may be used asboth gas inlet and gas outlet. Alternatively, multiple gas conduits maybe used, each functioning as a gas inlet or gas outlet.

[0022] If a single gas conduit is used, it may be attached to all gassources to be supplied to the chamber and a vacuum so the chamber may beevacuated. However, the vacuum may not be applied to the chamber at thesame time as gas is supplied. The vacuum is not necessary if inert gases(e.g. Ar) are used to flush the reaction chamber.

[0023] A gas inlet may be used to supply reducing or inert atmospheres,for example, H₂ and/or nitrogen or argon, to the chamber beforepyrolysis and to supply the gaseous carbonaceous material to bepyrolysed. These gases may be supplied through a single inlet or throughseparate inlets.

[0024] A gas outlet may be used to allow the exit of the unused gasesand byproducts of the pyrolysis reaction. A gas outlet may be attachedto a vacuum pump to allow evacuation of the reactor before theintroduction of a reducing and/or inert atmosphere. The gas outlet mayalso be attached to a device, such as a bubbler, to allow a slightpositive pressure of gas to be maintained in the chamber during thedeposition of carbon nanotubes.

[0025] In the process of the invention, the substrate may be anysubstrate-capable of withstanding the pyrolysis conditions employed andcapable of supporting carbon nanotube growth. Examples of suitablesubstrates include quartz glass, mesoporous silica, nanoporous alumina,ceramic plates, glass, graphite and mica. Preferably the substrate isordinary glass. Preferably the surface of the substrate upon which thecarbon nanotubes are grown is smooth.

[0026] The gaseous carbonaceous material may be any carbonaceouscompound or substance which may be gasified and which is capable offorming carbon nanotubes when subjected to pyrolysis. Examples of suchcompounds are alkanes, alkenes, alkynes and aromatic hydrocarbons, forexample, methane, ethylene, benzene or acetylene. Preferably thecarbonaceous material is acetylene.

[0027] Pyrolysis is performed in the presence of a catalyst. Thecatalyst may be any compound, element or substance suitable forcatalysing the conversion of a carbonaceous material to carbon nanotubesunder the pyrolysis conditions. Preferably the catalyst comprises atransition metal including Ni, Fe, Co, Al, Mn, Pd, Cr or alloys thereofin any suitable oxidation state. Most preferably, the catalyst comprisesNi. For example, the catalyst may be prepared frompolyvinylalcohol/Ni(NO₃)₂.6H₂O (PVA Ni.) Preferably, the surface of thesubstrate is coated with a substance from which the catalyst isprepared. For example, a spin-coated PVA Ni layer, subjected tooxidation at 500° C. for 30 minutes and reduction at 600° C. for 30minutes provided a catalyst coating showing strong adhesion onto a glasssubstrate, even when subjected to compressed air. Reduction of thecoating in the reactor is readily performed by supplying a mixture ofH₂/Ar to provide the catalyst-coated substrate. The substrate may thenbe maintained in an inert atmosphere, for example, nitrogen or argon, toprevent the catalyst being oxidised.

[0028] The pyrolysis conditions employed will depend on the nature ofthe gaseous carbonaceous material, the catalyst used, and the length anddensity of the carbon nanotubes required. It is possible to vary thepyrolysis conditions, such as temperature, time, catalyst, pressure orflow rate through the reactor to obtain carbon nanotubes havingdifferent characteristics.

[0029] Pyrolysis may be performed at temperatures above 300° C.Preferably in the process of the invention temperatures in the heatingzone are between 400° C. and 800° C. The selection of catalyst affectsthe temperature at which carbon nanotubes may be formed. The carbonformed during pyrolysis is then selectively deposited on the hot surfaceof the substrate in the heating zone, forming carbon nanotubes.Temperatures below 400° C. are demonstrated to be suitable for thenanotube growth with the ratio of carbon nanotubes to carbon nanofibre,their morphology and alignment depending on the conditions used.Surprisingly, it was found using the process of the present inventionthat well-aligned carbon nanotubes were easily formed well below thesoftening point of normal glass plates (ca. 640° C.). Within thetemperature ranging from 400° C. and 800° C., the nanotube deposition iscompleted within a couple of seconds to 20 minutes. The formation ofcarbon nanotubes is a typical transient reaction and its deposition ratecan be controlled by adjusting the pressure of the carbonaceous gas.With a low feed of carbonaceous gas, the carbon nanotube growth is fromedge to centre of the substrate. In contrast, a homogenous coating isseen over the substrate surface within a couple of seconds at a high gasfeed rate. Therefore, the deposition reaction can be region-specificallycontrolled by controlling the feed of the carbonaceous gas.

[0030] The carbon nanotubes produced by the process may be aligned ornon-aligned. Aligned or non-aligned carbon nanotubes may be selected forby varying temperatures, type of catalyst used and the density of thecatalyst coating on the substrate. For example, a low density ofcatalyst coating will favour non-aligned carbon nanotube growth whereasa high density of catalyst coating will favour aligned carbon nanotubegrowth.

[0031] The length of aligned carbon nanotubes may be varied over acertain range (from a sub-micrometre to several tens of micrometres) ina controllable fashion by changing the experimental conditions such asthe pyrolysis time and gas flow rate. The size and shape of the alignedcarbon nanotube film is, in principle, limited only by the size andshape of the substrate.

[0032] According to one embodiment of the invention, the reactionchamber may have a pre-heating zone, where the substrate may bepre-heated to a predetermined temperature before entering the localisedheating zone where carbon nanotube deposition occurs. The reactionchamber may also have a cooling zone where the substrate is cooled aftercarbon nanotube deposition is complete. In one embodiment of theinvention, the reactor includes a means of moving a substrate from thepre-heating zone to the localised heating zone and from the localisedheating zone to the cooling zone. For example, the substrate may sit ona transporting belt.

[0033] The reaction chamber of the present invention may be adapted tohave multiple localised heating zones by having multiple heatingelements located within the chamber. Carbon nanotubes may then bedeposited on multiple substrates simultaneously or in separate areas ofa larger substrate. In a reactor containing multiple localised heatingzones, a gas inlet may be attached to a gas distributor which allows thegaseous carbonaceous material to be supplied to each of the multiplelocalised heating zones simultaneously. Alternatively, the gaseouscarbonaceous materials may be supplied through an inlet at one end ofthe chamber and allowed to flow across each substrate and through eachlocalised heating zone sequentially.

[0034] The reactor of the present invention may be adapted to allowcontinuous carbon nanotube deposition on multiple substrates by having ameans of moving a substrate from a pre-heating zone to the localisedheating zone before carbon nanotube deposition and a means of moving asubstrate to a cooling zone after carbon nanotube deposition iscomplete. Continuous carbon nanotube production may be achieved bypre-heating a substrate or substrates in the pre-heating zone, movingthe substrate or substrates to the localised heating zone, synthesisingcarbon nanotubes on the substrate or substrates, moving the substrate orsubstrates into the cooling zone, removing the cooled substrate orsubstrates having a carbon nanotube film deposited on it/them from thereactor, and supplying new substrate or substrates to the pre-heatingzone. Generally only one step, e.g. pre-heating, carbon deposition orcooling, is performed at a certain part of the reactor at any one time.The process may be continuously repeated. The reduction of a substancefrom which the catalyst is prepared may also be performed in the reactorby supplying a mixture of Ar/H₂ to the chamber to provide the catalystcoated substrate.

[0035] In the continuous process, the substrate may be located in afirst chamber and then transported to a second chamber having apre-heating zone and/or localised heating zone and/or cooling zone.Advantageously, the reduction of a substance from which the catalyst isprepared may be performed in the first chamber to provide a catalystcoated substrate. The substrate may then be transported to thepre-heating zone or localised heating zone in the second chamber whenthe atmosphere in the first chamber has been flushed with inert gas. Ina preferred embodiment, the two chambers are connected and the catalystcoated substrate may pass from the first chamber to the pre-heating zoneor localised heating zone in the second chamber through a connectingdoor.

[0036] In a reactor adapted for use in the continuous process of theinvention, the localised heating zone and the cooling zone may be in thesame chamber. Alternatively, the cooling zone may be in a separatechamber from the localised heating zone. If the localised heating zoneand cooling zone are in separate chambers, the two chambers may beconnected such that the substrate in the localised heating zone may betransferred to the cooling zone via a connecting door after carbondeposition has occurred.

[0037] According to the present invention it is possible to preparemultilayer carbon nanotube materials by synthesising a first layer ofcarbon nanotubes on a substrate under a first set of pyrolysisconditions, and then synthesising a second layer of carbon nanotubes onthe nanotube coated substrate under a second set of pyrolysisconditions.

[0038] This process may be repeated until the desired number of carbonnanotube layers are present. Each layer of carbon nanotubes may bedeposited using the same or different pyrolysis conditions. Afterpreparation of the multilayer structure the carbon nanotube film may beremoved from the substrate using appropriate conditions.

[0039] It is also possible according to the present invention to preparehetero-structured multilayer carbon nanotube films by interposing layersof carbon nanotubes between layers of pyrolysis resistant materials, thecarbon nanotubes being generated in accordance with the process of thepresent invention.

[0040] The term “hetero-structured” as used herein refers to amultilayer structure which includes one or more carbon nanotube layerstogether with layers of other materials

[0041] The pyrolysis resistant material may be a metal, preferably Au,Pt, Ni, Cu, a semiconductor, TiO₂, MgO, Al₂O₃, ZnO, SnO₂, Ga₂O₃, In₂O₃,CdO or a polymer or any other pyrolysis resistant material that iscapable of supporting the nanotube growth.

[0042] The pyrolysis resistant material may be applied to the carbonnanotube coated substrate by any suitable means. Preferably metals areapplied by sputter-coating, polymers are applied by spin-casting andsemiconductors by sputter-coating or physical deposition.

[0043] As is evident from the above description, the process andapparatus of the invention allow the preparation of a large variety ofcarbon nanotube films and structures. It is also possible to providepatterned layers using appropriate masking and etching techniques. Atlower temperatures it may be possible to use the reactor of theinvention to prepare carbon nanofibres or mixtures of carbon nanotubesand nanofibres.

[0044] The materials produced by the present invention may be used inthe construction of devices for practical applications in many fieldsincluding electron emitters, field-emission transistors, electrodes forphotovoltaic cells and light emitting diodes, optoelectronic elements,bismuth actuators, chemical and biological sensors, gas and energystorages, molecular filtration membranes and energy-absorbing materials.

[0045] The invention can be more fully understood from the followingdetailed description of FIG. 1 and the examples. It should be understoodthat the examples and Figures described are only for illustrationpurposes, which does not intend to constitute a limitation on theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIG. 1AA is a diagrammatic side-view representation of a pyrolysisflow reactor of the invention having a gas inlet at one end of thechamber, and a gas outlet at the other.

[0047]FIG. 1AB is a diagrammatic side view representation of a pyrolysisflow reactor of the invention having a gas inlet positioned above thelocalised heating zone and gas outlets positioned on the respective endsof the reaction chamber.

[0048]FIG. 1AC is a diagrammatic end view representation of a pyrolysisreactor of the invention having a gas inlet positioned above thelocalised heating zone and a gas outlet positioned below.

[0049]FIG. 1BA is a diagrammatic side view representation of a pyrolysisreactor of the invention having multiple localised heating zones.

[0050]FIG. 1BB is a diagrammatic top view representation of a heatingelement supporting multiple substrates.

[0051]FIG. 1CA is a diagrammatic side view representation of a pyrolysisreactor of the invention having a pre-heating zone, a localised heatingzone and a cooling zone and wherein inlet for supplying the gaseouscarbonaceous material is positioned above the localised heating zone.

[0052]FIG. 1CB is a diagrammatic top plan view of a heating elementhaving a pre-heating zone, a localised heating zone and a cooling zone.

[0053]FIG. 1D is a diagrammatic side plan view of a reactor of theinvention in which a substrate may be moved from the pre-heating zone tothe localised heating zone and finally to the cooling zone beforeremoval from the reactor.

[0054]FIG. 1EA is a series of diagrammatic side plan views of a heatingelement designs suitable for use in the reactor of the invention.

[0055]FIG. 1EB is a diagrammatic top plan view of a heating elementsuitable for use in the reactor of the invention.

[0056]FIG. 1AA is a diagrammatic representation of a pyrolysis reactorof the invention in which a substrate (1) has been positioned on theheating element (2). The heating element (2) and substrate (1) arelocated within a reaction chamber (7) defined by chamber walls (3). Asheet of conducting material (4) is positioned between the substrate (1)and the heating element (2) in order to provide homogenous heating ofthe substrate (1). A gas inlet (5) is positioned at one end of thereaction chamber and a gas outlet (6) is positioned at the opposing endof the reaction chamber such that any gas introduced into the reactionchamber (7) flows from one end of the chamber to the other, flowing overthe substrate (1).

[0057]FIG. 1AB is a diagrammatic representation of a pyrolysis reactorof the invention similar to that shown in FIG. 1AA but having a gasinlet (5) positioned above the heating element (2) upon which thesubstrate (1) is supported. The gas is suppled directly into thelocalised heating zone (8). Gas outlets (6) are located at each end ofthe chamber.

[0058]FIG. 1AC is a diagrammatic end view representation of a pyrolysisreactor of the invention similar to that shown in FIG. 1AA or 1AB buthaving a gas outlet (6) located below the localised heating zone (8).

[0059] In operation, the reactor shown in FIGS. 1AA to 1AC is flushedwith an inert gas such as N₂ or Ar, or is evacuated by means of a vacuumpump connected to the gas outlet (6). A mixture of H₂ and Ar isintroduced into the reaction chamber (7) to ensure the catalyst is in areduced state. The heating element (2) is heated to the requiredtemperature which heats the substrate (1) to the required temperature.When the catalyst has been reduced, a flow of Ar gas is maintained toensure an inert atmosphere in the chamber. A gaseous carbonaceousmaterial, such as ethylene, is introduced into the reaction chamber (7)through the gas inlet (5). Pyrolysis of the carbonaceous material occursin the localised heating zone (8) and carbon is deposited on the hotsurface of the substrate (1) to produce a carbon nanotube layer. Thesubstrate (1) is then allowed to cool.

[0060]FIG. 1BA is a diagrammatic side view representation of a pyrolysisreactor of the invention in which multiple substrates (1) are positionedon a heating element (2). The heating element (2) and the substrates arelocated within a reaction chamber (7) defined by chamber walls (3). Thegas inlet (5) is connected to a gas distributor (9) that allows thecarbon-containing material to be simultaneously introduced into multiplelocalised heating zones (8). The gas outlet (6) is positioned at one endof chamber (7) and a connection to a vacuum pump (10) is positioned atthe opposing end of chamber (7).

[0061]FIG. 1BB is a diagrammatic top plan view representation of theheating element (2) of the reactor shown in FIG. 1BA. Multiplesubstrates (1) are positioned on the heating element (2).

[0062] In operation, the reactor shown in FIG. 1BA is evacuated by avacuum pump connected to outlet (10) and a mixture of H₂ and Ar isintroduced into the reaction chamber (7). The heating element (2) isheated to the required temperature which heats the multiple substrates(1) to the required temperature. When the catalyst has been reduced, apositive pressure of inert atmosphere (Ar) is maintained in the reactionchamber (7). A gaseous carbonaceous material is introduced into multiplelocalised heating zones (8) from the gas inlet (5) by means of a gasdistributor (9). Pyrolysis of the carbonaceous material occurs in themultiple localised heating zones (8) and carbon is deposited on the hotsurfaces of the multiple substrates (1) to produce a carbon nanotubelayer on each of the multiple substrates (1). The substrates are thenallowed to cool. Multiple substrates are simultaneously coated with alayer of carbon nanotube in this reactor.

[0063]FIG. 1CA is a diagrammatic side view representation of a pyrolysisreactor of the invention in which a substrate (1) is positioned on theheating element (2). The heating element (2) and the substrate (1) islocated within the reaction chamber (7) defined by chamber walls (3).The heating element (2) is divided into zones having differenttemperatures, a pre-heating zone (11), a localised heating zone (8) anda cooling zone (12). The gas inlet (5) is positioned above the localisedheating zone (8) and distributes the gaseous carbonaceous materialevenly over the substrate (1) when it is in the localised heating zone(8). The gas outlet (6) is positioned at one end of the chamber (7) anda connection to a vacuum pump (10) is positioned at the opposing end ofthe chamber (7).

[0064]FIG. 1CB is a diagrammatic top plan view representation of aheating element (2) of the reactor shown in FIG. 1CA. A substrate (1) isinitially positioned in the pre-heating zone (11), then the substrate ismoved to the localised heating zone and finally the substrate is movedto the cooling zone (12).

[0065] In operation, a substrate (1) is pre-heated to a predeterminedtemperature in the pre-heating zone (11). The substrate (1) is thenmoved to the localised heating zone (8) and is heated to pyrolysistemperature. A gaseous carbonaceous material is introduced into thelocalised heating zone (8) through gas inlet. The gaseous carbonaceousmaterial is pyrolysed and carbon nanotubes are deposited on the surfaceof the substrate (1). The substrate (1), upon which carbon nanotubeshave been deposited, is moved to the cooling zone (12) and allowed tocool before being removed from the reactor. The reactor may also includea mechanism for transferring the cooled substrate (1) having a film ofcarbon nanotubes out of the reactor and a mechanism for positioning anew substrate in the pre-heating zone. The reactor may also include amechanism which allows transfer of a substrate from one zone to the nextwithin the chamber. (7) The process may be performed in a continuousmanner, for example, the substrate may sit on a transporting belt. Sucha device is also illustrated in FIG. 1D.

[0066] FIGS. 1EA to 1ED show a series of heating element (2)/conductingmaterial (4)/substrate (1) configurations which may be useful in thereactor of the invention. In FIG. 1EA, the sheet of conducting material(4) and the substrate (1) are embedded in the heating element (2). InFIG. 1EB, the sheet of conducting material (4) is embedded in theheating element (2) and a substrate (1) which is smaller than theheating element (2) is placed in the heating element (2) on theconducting metal sheet (4). In FIG. 1EC, the surface of the heatingelement (2) is flat and a substrate (1) having a smaller surface thanthe heating element (2) is placed on a sheet of conducting material (4)also having a smaller surface than the heating element (2). The sheet ofconducting material (4) and the substrate (1) protrude above the surfaceof the heating element (2). In FIG. 1ED, the heating element (2) iscoated with a conducting material (4) across its entire surface and asubstrate (1) is positioned on top of the conducting material coating.The configuration in FIG. 1ED is the most suitable for use in a reactorwhere continuous carbon nanotube deposition occurs, for example, thereactor shown in FIG. 1C or FIG. 1D.

[0067]FIG. 1EE is a diagrammatic top plan view of a preferred heatingelement (2) having a substrate (1) positioned on it. The heating element(2) has a sheet of conducting material (4) placed on it to providehomogenous temperature distribution and a resistant wire (13) issupported within the heating element to heat the heating element.

EXAMPLES Example 1 Preparation of Non-Aligned Carbon Nanotubes

[0068] A glass substrate was spin-coated with a PVA Ni layer (100 mMNi(NO₃)₂.6H₂O and 3 wt % PVA) to provide a catalyst for deposition ofcarbon nanotubes. The coated substrate was oxidised at 500° C. for 30minutes and reduced at 600° C. for 30 minutes. After such treatment, thecatalyst coating was strongly adhered to the substrate. Thecatalyst-coated substrate was placed in the reactor as shown in FIG. 1Aand the atmosphere in the reaction chamber was replaced with H₂/Ar toensure the catalyst was in a reduced state. The substrate was heated at650° C. on the heating element and acetylene/Ar gas (V:V=1:3),introduced at a total flow rate of 60 ml/min for a time of 3 minutes,was pyrolysed resulting in the deposition of non-aligned carbonnanotubes on the catalyst-coated substrate.

Example 2 Preparation of Aligned Carbon Nanotubes

[0069] Aligned carbon nanotubes were prepared by the same method asapplied in Example 1, but at a lower temperature (440° C.) was used. Theresulting carbon nanotubes align almost normal to the substrates surfaceand are densely packed with a fairly uniform tubular length of ca. 1 μm.

The claims defining the invention are as follows:
 1. A process forpreparing carbon nanotubes comprising: locating a substrate capable ofsupporting carbon nanotube growth in a localised heating zone within areaction chamber, said localised heating zone being provided by aheating element located within said reaction chamber, passing a gaseouscarbonaceous material into said reaction chamber such that the gaseousmaterial passes over and contacts said substrate in the localisedheating zone, whereby said gaseous material undergoes pyrolysis underthe influence of said heat to form carbon nanotubes on said substrate.2. A process according to claim 1 wherein the localised heating zone hasa temperature greater than 300° C.
 3. A process according to claim 2wherein the localised heating zone has a temperature between 400° C. and800° C.
 4. A process according to claim 1 wherein the substrate isquartz glass, mesoporous silica, nanoporous alumina, a ceramic plate,glass, graphite or mica.
 5. A process according to claim 4 wherein thesubstrate is glass.
 6. A process according to claim 1 wherein thegaseous carbonaceous material is selected from an alkane, alkene, alkyneor aromatic hydrocarbon.
 7. A process according to claim 6 wherein thegaseous carbonaceous material is selected from methane, ethylene,benzene or acetylene.
 8. A process according to claim 7 wherein thegaseous carbonaceous material is acetylene.
 9. A process according toclaim 1 wherein pyrolysis of the carbonaceous material occurs in thepresence of a catalyst.
 10. A process according to claim 9 wherein thecatalyst is coated on the substrate.
 11. A process according to claim 9or 10 wherein the catalyst comprises a transition metal selected fromNi, Fe, Co, Al, Mn, Pd, Cr or alloys thereof.
 12. A process according toclaim 11 wherein the catalyst comprises Ni.
 13. A process according toclaim 1 wherein the pyrolysis conditions are controlled to providealigned carbon nanotubes.
 14. A process according to claim 1 wherein thepyrolysis conditions are controlled to provide non-aligned carbonnanotubes.
 15. A process according to claim 1 wherein the pyrolysisconditions are controlled to provide homogeneous carbon nanotube growthon the substrate.
 16. A process according to claim 1 wherein thepyrolysis conditions are controlled to provide patterned carbon nanotubegrowth on the substrate.
 17. A process for preparing multilayer carbonnanotube materials comprising: (a) synthesising a first layer of carbonnanotubes on a substrate under a first set of pyrolysis conditions toprovide a nanotube coated substrate; (b) synthesising a second layer ofcarbon nanotubes on the nanotube coated substrate under a second set ofpyrolysis conditions, wherein at least one of steps (a) and (b) isperformed using a process of claim
 1. 18. A process according to claim17 wherein step (b) is repeated at least once.
 19. A process accordingto claim 17 wherein the pyrolysis conditions of step (a) are the same asthe pyrolysis conditions of step (b).
 20. A process according to claim17 wherein the pyrolysis conditions of step (a) are different from thepyrolysis conditions of step (b).
 21. A process for the preparation of ahetero-structured multilayer carbon nanotube film comprising: (a)synthesising a first layer of carbon nanotubes on a substrate under afirst set of pyrolysis conditions to provide a nanotube coatedsubstrate; (b) coating a layer of pyrolysis resistant material onto thenanotube coated substrate to provide a hetero-structured multilayersubstrate; (c) synthesising a second layer of carbon nanotubes on thehetero-structured multilayer substrate under a second set of pyrolysisconditions, wherein at least one of steps (a) and (c) is performed usinga process of claim
 1. 22. A process according to claim 21 wherein steps(b) and (c) are repeated at least once.
 23. A process according to claim21 wherein the pyrolysis conditions of step (a) are the same as thepyrolysis conditions of step (c).
 24. A process according to claim 21wherein the pyrolysis conditions of step (a) are different from thepyrolysis conditions of step (c).
 25. A process according to claim 21wherein the pyrolysis resistant material is a metal, a semiconductor ora polymer.
 26. A process according to claim 25 wherein the pyrolysisresistant material is a metal.
 27. A reactor for preparing carbonnanotubes comprising: a reaction chamber, at least one support meanslocated within said reaction chamber capable of supporting a substrate,said substrate being capable of supporting carbon nanotube growth, atleast one heating element located within said reaction chamber capableof providing localised heating to said substrate within said reactionchamber, means for passing a gaseous carbonaceous material into saidreaction chamber such that it passes over and contacts said substrate.28. A reactor according to claim 27 wherein the reaction chamber isformed from metal, glass, plastic or a combination thereof.
 29. Areactor according to claim 28 wherein the reaction chamber is formedfrom glass or comprises at least one glass panel.
 30. A reactoraccording to claim 27 wherein the heating element comprises resistantwires, an induction field, microwave radiation or infrared radiation.31. A reactor according to claim 27 wherein the heating element islocated within the substrate support.
 32. A reactor according to claim31 wherein the heating element and substrate support comprise a ceramicplate into which resistant wires have been inserted.
 33. A reactoraccording to claim 27 wherein the means for passing a gaseouscarbonaceous material into the reaction chamber is at least one gasconduit.
 34. A reactor according to claim 33 wherein the at least onegas conduit is located above the substrate.
 35. A reactor according toclaim 33 wherein the at least one gas conduit is located to allow thegaseous carbonaceous material to flow across the surface of thesubstrate.
 36. A reactor according to claim 27 comprising multiplesupport means.
 37. A reactor according to claim 27 comprising multipleheating elements.
 38. A reactor according to claim 27 further comprisinga pre-heating zone.
 39. A reactor according to claim 38 wherein thepre-heating zone is located in a separate chamber from the reactionchamber.
 40. A reactor according to claim 27 further comprising acooling zone.
 41. A reactor according to claim 40 wherein the coolingzone is located in a separate chamber from the reaction chamber.
 42. Areactor according to claim 27 further comprising a means of transferringa substrate from a pre-heating zone to a support means and/or from thesupport means to a cooling zone.
 43. Carbon nanotubes prepared by theprocess of any one of claims 1 to
 16. 44. Multilayer carbon nanotubesprepared by the process of any one of claims 17 to
 20. 45.Hetero-structured multilayer carbon nanotube films prepared by theprocess of any one of claims 21 to 26.